The present invention relates to a magnetic ferrite material, particularly to an MnZn-based magnetic ferrite material for a transformer to be used in the magnetic core of a switching power supply, and the like and a manufacture method thereof.
An MnZn-based magnetic ferrite material has heretofore been used mainly as a transformer material for a communication apparatus and a power supply. Different from other magnetic ferrite materials, the MnZn-based magnetic ferrite material is characterized in that it has a high saturation magnetic flux density, permeability is also high, and power loss is small during the use as the transformer, and the addition of SiO2, CaO can lower the power loss. Moreover, various additives are studied for a purpose of further reduction of the power loss.
However, the power loss is largely influenced by a slight amount of impurities contaminated during the manufacture process of the magnetic ferrite material, or a slight amount of impurities contained in a raw material. Therefore, the development of an MnZn-based magnetic ferrite material which stably realizes a low power loss has been desired.
The present invention has been developed in consideration of the above-described situation, an attention is given to that the non-uniform dispersion of a Ca component in the grain boundary of an MnZn-based magnetic ferrite material obstructs power loss reduction, and an object of the present invention is to provide an MnZn-based magnetic ferrite material which can realize a low power loss, and a method of manufacturing the magnetic ferrite material.
To achieve this and other objects, according to the present invention, there is provided a magnetic ferrite material which is obtained by calcining a raw material, forming a calcined powder into a desired shape and sintering and which contains Fe2O3, MnO and ZnO as main components, and in the constitution, the coefficient of variation (CV value) of the content of a Ca component precipitated along a grain boundary is in a range of 1 to 60%.
Moreover, in preferable embodiment, the magnetic ferrite material of the present invention is constituted so that the content of the Ca component is in a range of 200 to 1200 ppm. The Ca component is included in the calcined powder.
According to the present invention, there is provided a magnetic ferrite material manufacture method comprising the steps of: calcining a raw material containing Fe2O3, MnO and ZnO as main components to obtain a calcined powder in which the content of an S component is in a range of 1 to 200 ppm; and forming the calcined powder into a desired shape and sintering.
Moreover, in preferable embodiment, the magnetic ferrite material manufacture method of the present invention is constituted so that desulfurization is performed in the step of forming the calcined powder.
In the present invention, since the coefficient of variation (CV value) of the content of the Ca component precipitated along the grain boundary of the magnetic ferrite material is in the range of 1 to 60%, the Ca component as the additive is uniformly present along the grain boundary. Therefore, a high resistance layer uniformly surrounds grains, an eddy-current loss decreases, and the magnetic ferrite material with a low power loss can be obtained.
Moreover, by setting the content of the S component of the calcined powder to be in a predetermined range, the formation of CaSO4 is depressed, and the Ca component as the additive easily forms a liquid phase with ferrite, so that the Ca component is prevented from being non-uniformly dispersed in the grain boundary of the magnetic ferrite material; and the magnetic ferrite material with the low power loss can be manufactured.
An embodiment of the present invention will next be described.
According to the present invention, there is provided a magnetic ferrite material which is obtained by calcining a raw material, forming a calcined powder into a desired shape and sintering and which contains Fe2O3, MnO and ZnO as main components. Moreover, the coefficient of variation (CV value) of the Ca content precipitated along the grain boundary of magnetic ferrite is in a range of 1 to 60%. In the present invention, since the CV value is in the range of 1 to 60%, the Ca component is prevented from being non-uniformly dispersed in the grain boundary of the magnetic ferrite material.
Here, the coefficient of variation (CV value) of the Ca content precipitated along the grain boundary will be described. In the present invention, composition is analyzed by a transmission electron microscope, and a value calculated by s/xxc3x97100(%) from a standard deviation s and average value x of the analyzed value is used as the coefficient of variation (CV value). In the above-described composition analysis, the diameter of a measurement area of one spot by an electron beam is set to 25 nm, the center of the electron beam spot is aligned with the center of the grain boundary, and the Ca content is measured in at least ten spots around one grain.
The coefficient of variation (CV value) of the Ca content with a larger numeric value means that the Ca component present along the grain boundary is non-uniform. In the present invention, by defining the CV value to be 60% or less, the Ca component is uniformly present along the grain boundary, a high resistance layer uniformly surrounds he grain, an eddy-current loss decreases, and the magnetic ferrite material with a low power loss (300 kW/m3 or less) can be realized.
The Ca content of the magnetic ferrite material of the present invention is preferably in a range of 200 to 1200 ppm. When the Ca content exceeds 1200 ppm, abnormal grain growth occurs and electromagnetic properties are deteriorated, and when the content is less than 200 ppm, a sufficient power loss reduction is not achieved. Additionally, the Ca component can be analyzed by dissolving the ground sample in aqua regia (nitric acid 1: hydrochloric acid 3) and using ICP emission spectroscopy.
According to the present invention, there is provided a magnetic ferrite material manufacture method comprising: first calcining a raw material containing Fe2O3, MnO and ZnO as main components to obtain a calcined powder in which the content of an S component is in a range of 1 to 200 ppm. Subsequently, by forming the calcined powder into a desired shape and sintering, a magnetic ferrite material is obtained. In the present invention, by defining the content of the S component of the calcined powder, the segregation of the Ca component based on the formation of CaSO4 is depressed, and the magnetic ferrite material with the low power loss can be manufactured.
When the content of the S component of the calcined powder exceeds 200 ppm, calcium sulfate (CaSO4) is formed, the Ca component does not easily form a liquid phase with ferrite, and the non-uniform dispersion of the Ca component undesirably becomes remarkable. When the S content is less than 1 ppm, the material cost increase by the high purification of the raw material is undesirably caused.
In order to obtain the calcined powder so that the S content is in a range of 1 to 200 ppm, there are measures of: (1) using a raw material, a defoamer, a dispersant, and the like which contain a small amount of S, or no S component; and (2) reducing the amount of S element in the material by a desulfurizing process when the S component is contaminated in the material. The latter desulfurizing method is not particularly limited. When S in the material is easily decomposed in a low temperature, desulfurization can be performed by wet grinding, then drying a material slurry and further heating the obtained calcined powder to a temperature at which S is decomposed. Subsequently, by further grinding or cracking the calcined powder, adjusting the grain size of the calcined powder, and shaping and sintering, the ferrite material may be obtained. On the other hand, when S is not easily decomposed, such as CaSO4, the desulfurization can be performed, for example, by drying the wet-ground material slurry on a heated plate of a metal (e.g., stainless steel, iron, titanium, and the like). Furthermore, when S is relatively easily dissolved in water, the desulfurization can be performed by filtering the wet-ground material slurry, removing the solution containing much S, and drying the slurry.
The S component in the calcined powder can be measured by grinding and then calcining/oxidizing the sample, and using an infrared detector to analyze converted SO2.
Additionally, the calcined powder preferably contains an Si component in a range of 60 to 200 ppm, and a Ca component in a range of 200 to 1200 ppm for a purpose of power loss reduction. When the content of the Si component exceeds 200 ppm, in the sintering process the abnormal grain growth occurs and electromagnetic properties are deteriorated, and when the content is less than 60 ppm, a sufficient power loss reduction is not achieved. Moreover, when the content of the Ca component exceeds 1200 ppm, in the sintering process the abnormal grain growth occurs and the electromagnetic properties are deteriorated, and when the content is less than 200 ppm, the sufficient power loss reduction is not achieved.
To aim at the low power loss, as the additives other than SiO2, CaCO3, a slight amount of additives contributing to the low power loss, such as Nb2O5, ZrO2, V2O5, Ta2O5, may be added. In this case, as the range of an additive amount, about 50 to 500 ppm is preferable for Nb2O5, V2O5, Ta2O5, and about 10 to 450 ppm is preferable for ZrO2.
After forming the calcined powder into the desired shape in which the content of the S component is in a range of 1 to 200 ppm, the sintering can be performed, for example, at a temperature rise speed of about 100xc2x0 C./hour in a range of 1200 to 1400xc2x0 C. Moreover, in the subsequent cooling, the powder is cooled to a normal temperature at a cooling speed of 50 to 500xc2x0 C./hour.
Concrete examples will next be described to describe the present invention in more detail.